Various devices and prostheses have been proposed to correct and/or stabilize spinal injuries or deformities. Such devices include artificial spinal discs, nuclei etc. Such devices serve to replace existing damaged or diseased portions of the spine. In some cases however, it is desirable or necessary to fuse spinal vertebrae so as to prevent or reduce any movement. Such fixation devices commonly utilize pedicle screws that are implanted into the pedicles of vertebrae and serve as anchors for other prosthetic devices. FIGS. 1 and 2 illustrate a vertebral segment 100 with pedicles 102a and 102b that extend from the vertebral body 101. FIG. 2 illustrates the placement of pedicle screws 200 as known in the art. Such pedicle screws 200 have a threaded portion 208 that is screwed into the pedicle and head portions 204 and 206 respectively that connect to other fixation devices such as a rod 206.
Pedicle screw fixation systems have been used in providing spinal stabilization and spinal fusion in patients with a variety of conditions such as degenerative spondylolisthesis, isthmic spondylolisthesis, fusion after decompression, spinal fractures, and surgically repaired spinal pseudoarthroses. The advent of rigid pedicle screw/rod fixation devices has led to a dramatic increase in the rate of arthrodesis (i.e. the surgical fusion of a joint) particularly for the treatment of degenerative disc disease and spondylolisthesis. In addition to higher rates of arthrodesis, rigid instrumentation has enabled surgeons to maintain, improve, or fully reduce spondylolisthesis outright, and these devices have allowed for very aggressive strategies for decompression.
As shown in FIG. 2, typical pedicle screw fixation systems as known in the art are multi-component devices consisting of solid rods 206 that are longitudinally interconnected and anchored to adjacent vertebrae using pedicle screws such as pedicle screw 200. The screws and other components are generally made of stainless steel, titanium or other acceptable implantable material, typically metal alloys. The surgeon selects from among these components to construct a system suitable for a patient's anatomical and physiological requirements. Pedicle screws are similar to the screws used in long bones.
During implantation, pedicle screws are inserted into channels that are drilled or otherwise formed through the cancellous central axis of each vertebral pedicle 102a and 102b. The longitudinal connecting rods 206 usually span and brace two or more vertebrae and, as mentioned above, are connected to the screws 200. Each vertebra typically receives a pedicle screw in both pedicles. The connecting rods 206 are provided in pairs with each of the rods extending over one side of the spine.
The screws hold their purchase within the bones through several mechanisms. One of the main sources of pullout resistance is obtained by the use of screw threads. The use of threads allows for better fixation due to increased contact area with the surrounding bone material. It is understood in the current art that placement of the screw in a manner such that it is directed towards the median plane of the vertebrae improves pullout resistance by allowing the screw to interact with a greater amount of bone material.
Insufficient resistance against pullout of the bone screws is a recognized problem with current bone screws. This problem is faced in cases of poor bone quality such as in those patients with osteoporosis. Fixation of a screw into bone is directly related to the amount of contact area between the bone and the screw, as well as the quality of that contact. Therefore, the more direct contact there is between the bone and the surface of the screw, the better the purchase and fixation. A long screw with a large diameter will provide better fixation than a short screw with a lesser diameter as a result of the larger surface contact area of the larger screw. Also, the density of the bone determines the actual real contact surface between screw and bone, as bone with a high density will have more bone material in direct contact with the available screw surface than bone with lower density. Thus, in patients with osteoporosis where the bone mineral density is low, there is less surface contact between the screw and bone than in patients with normal bone mineral density.
Screw loosening as a result of constant back and forth toggling forces acting on the screw is also a cause for screw pullout. These forces can occur during regular flexion and extension motions of the spine (Chao, C. K. et al. Increasing Bending Strength and Pullout Strength in Conical Pedicle Screws: Biomechanical Tests and Finite Element Analyses. J. Spinal Disorders & Techniques. 2008. 21 (2): 130-138, 2008).
Examples of known pedicle screws are provided in U.S. Pat. Nos. 4,887,596 and 5,207,678. Some more recent screws and screw systems have been proposed to address specific issues. For example, a cannulated pedicle screw is provided in US publication number US2007/0299450. In this reference, the pedicle screw is provided with a central cannula or canal having an opening at the distal end of the screw. Once implanted, bone cement is injected into the cannula and into the joint between the screw and the bone.
U.S. Pat. No. 7,037,309 provides another cannulated pedicle screw having a self-tapping distal tip. A screw of this type avoids the need for boring hole prior to insertion of the screw.
US publication numbers US2005/0182409 and US2008/0015586 teach a device for dynamic stabilization of the spine and are directed to the problem of shear stresses on pedicle screws. In these references, the devices include pedicle screws that are provided with a head that connects to moveable elements. In the course of regular motion, such elements are adapted to absorb compressive or expansive forces and to thereby reduce the amount of stresses translated to the screws. The moveable elements are often complicated devices as compared to the commonly known rods.
There is a need for a bone screw that resists pullout.